The present invention relates to an ultrasonic sensor with at least two ultrasonic converters.
Ultrasonic flow sensors are used, in particular, to measure the volume flow, mass flow, or the flow rate of a gaseous or liquid medium that flows through a pipeline. A known type of ultrasonic flow sensor includes two ultrasonic converters located such that they are offset in the direction of flow, each of which generates ultrasonic signals and transmits them to the other ultrasonic converter. The ultrasonic signals are received by the other converter and are evaluated electronically. The difference between the transit time of the signal in the direction of flow and the transit time of the signal in the opposite direction is a measure of the flow velocity of the fluid.
FIG. 1 shows a typical design of an ultrasonic flow sensor 6 with two ultrasonic converters A, B, which are located inside a pipeline 3 and are diametrically opposed at a distance L from each other. A fluid 1 flows in pipeline 3 with a velocity v in the direction of arrow 2. Measurement path L is tilted relative to flow direction 2 at an angle α. While a measurement is being carried out, ultrasonic converters A, B send ultrasonic pulses to each other. The signals are decelerated or accelerated, depending on the direction of the flow. The transit times of the ultrasonic signals are a measure of the flow rate to be determined.
FIG. 2 shows a greatly simplified schematic depiction of associated transmitting and receiving circuit 4. The two ultrasonic converters A, B are activated by an oscillator with a burst having a specified clock frequency 8 (e.g., a square-wave signal). Ultrasonic signals 7 generated as a result (only the envelopes of the signals are shown here) travel along measurement path L and are detected by the other ultrasonic converter A, B. Transit time t12 or t21 of signals 7 is measured.
When the medium is at rest, the differential transit time should be equal to zero. In reality, deviations always occur, however, which are based in particular on tolerances of the two ultrasonic converters A, B. As a result, even when medium 1 is at rest, a differential transmit time not equal to zero is measured (a “zero flow error”). Due to temperature and aging influences, this offset may change over the course of time, thereby resulting in measurement errors.